22 2 NO and ART
2.3 Interactions of Heme with NO and ART
GC is a major receptor for NO binding, by which the enzyme is activated to
catalyze the generation of cGMP, thereby playing a pivotal role in cellular func-
tion and metabolic regulation (Hobbs and Stasch 2010 ). On the other hand, NO
behaves as a reversible inhibitor of COX to exert a signaling effect (Boveris et al.
2010 ). Both GC and COX are hemoproteins, whose prosthetic heme moieties are
targeted by NO. In theory, all kinds of hemoproteins can interact with NO through
the heme-NO interaction albeit resulting in either activation or inhibition.
Based on their functions, hemoproteins can be classified into four subtypes:
(1) O 2 transport: hemoglobin, myoglobin, neuroglobin, cytoglobin, and leghemo-
globin; (2) electron transfer: cytochrome a, cytochrome b, and cytochrome c; (3)
catalysis: COX, NOS, CAT, POX, cytochrome P450s, ligninase, and tryptophan
oxygenase; (4) sensory: GC, FixL (an O 2 sensor), and CooA (a carbon oxide
sensor).
2.3.1 Activation of GC by the NO-Heme Interaction
The hemoprotein GC is a heterodimer composed of one α subunit and one heme-
binding β subunit. The mammalian GC contains one heme per heterodimer. NO
binding to heme results in the activation of a C-terminal catalytic domain and the
production of cGMP. NO leads to at least 200-fold increase of GC activity (Wolin
et al. 1982 ), in which NO displaces the axial histidine (His^105 ) by direct competi-
tion at the proximal side of heme (Lawson et al. 2000 ). However, the exact mecha-
nism by which NO-mediated changes in the heme coordination resulting in the
activation of GC has not yet been established.
Under oxidative stress conditions, GC can be oxidized and lose its heme. A
heme-free enzyme no longer responds to NO, but it can still respond to other acti-
vators because they can bind to the empty pocket of heme in a similar manner
with NO. In addition, GC contains an allosteric site to which some stimulators can
bind. These allosteric agents can potentate the NO-GC signaling, thereby allowing
even a suboptimal NO level also maximally activating GC.
2.3.2 Inhibition of COX by the NO-Heme Interaction
The hemoprotein COX is a large transmembrane protein complex found in bac-
teria and mitochondria in eukaryotes. It is a downstream enzyme complex on the
respiratory chain. It receives an electron from each of four cytochrome c, and
transfers them to O 2. COX accepts four protons from the inner aqueous phase,